Fix #34938: Not freeing dynstr_cstring memory blocks when setting shape keys

In fact almost any rna pointer-to-string had a memory leak.

Patch by Jakub Zolcik (sftd) with minor modification.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@55926 954f8c5b-7b00-dc11-b283-0030488c597c